This invention relates to reducing energy loss and noise in power converters.
As shown in FIGS. 1 and 2, in a typical PWM non-isolated DC-to-DC shunt boost converter 20 operated in a discontinuous mode, for example, power is processed in each of a succession of power conversion cycles 10. During a power delivery period 12 of each power conversion cycle 10, while a switch 22 is open, power received at an input voltage Vin from a unipolar input voltage source 26 is passed forward as a current that flows from an input inductor 21 through a diode 24 to a unipolar load (not shown) at a voltage Vout. Vout is higher than the input voltage, Vin.
FIGS. 2A and 2B show waveforms for an ideal converter in which there are no parasitic capacitances or inductances and in which the diode 24 has zero reverse recovery time. During the power delivery period 12, the current in the inductor falls linearly and reaches a value of zero at time tcross. At tcross, the ideal diode immediately switches off, preventing current from flowing back from the load towards the input source, and the current in the inductor remains at zero until the switch 22 is closed again at the next time ts1off. Thus, no energy is stored in the inductor 21 between times tcross and ts1on.
During another, shunt period 14 of each cycle, while switch 22 is closed, the voltage at the left side of the diode (node 23) is grounded, and no current flows in the diode. Instead, a shunt current (Is) is conducted from the source 26 into the inductor 21 via the closed switch 22. In a circuit with ideal components, the current in the inductor would begin at zero and rise linearly to time ts1off, when switch 22 is turned off to start another power delivery period 12.
In a non-ideal converter, in which there are parasitic circuit capacitances and the diode is non-ideal (e.g., for a bipolar diode there will be a reverse recovery period and for a Schottky diode there will be diode capacitance), an oscillatory ringing will occur after tcross.
In one example, waveforms for a non-ideal converter of the kind shown in FIG. 1 are shown in FIGS. 2C and 2D. Because of the reverse recovery characteristic of the diode, the diode does not block reverse current flow at time tcross. Instead, current flows in the reverse direction through the diode 24 and back into the inductor 21 during a period 18. At time tdoff, the diode snaps fully off and the flow of reverse current in the diode goes to zero.
Because of the reverse flow of current in the diode during the diode recovery period, energy has been stored in the inductor as of the off time tdoff (the “recovery energy”). In addition, parasitic circuit capacitances (e.g., the parasitic capacitances of the switch 22, the diode 24, and the inductor 24, not shown) also store energy as of time tdoff (e.g., the parasitic capacitance of switch 22 will be charged to a voltage approximately equal to Vout).
After time tdoff, energy is exchanged between the inductor and parasitic capacitances in the circuit. As shown in FIGS. 2C and 2D, the energy exchange causes oscillatory ringing noise in the circuit. Furthermore, the presence of oscillatory current will generally result in energy being dissipated wastefully in the circuit at the start of the next shunt period when the switch is closed at time ts1on. The energy loss can amount to several percent of the total energy processed during a cycle.